1. Field of the Invention
The invention concerns a decontamination process for detaching or dissolving a contamination layer covering a metallic surface, which layer contains the corrosion products of the metal in question and/or substances which have come into contact with the metallic surface or the contamination layer and have been deposited or absorbed or adsorbed there and/or substances that have diffused through the metal to the metal surface and/or chemical transformation or decomposition products of the said substances, by single or multiple dissolving-off with oxidising and/or reducing solutions, particularly for the decontamination of metallic surfaces in coolant circuits of nuclear reactors or technical devices coupled thereto and having coolant flowing through them, as well as to the use of this process for chromium and/or cobalt containing metals.
2. Description of the Prior Art
Decontamination processes of this type are known, e.g. from the textbook by J. A. Ayres, "Decontamination of Nuclear Reactor and Equipment", Ronald Press Co., New York 1970, wherein several processes used hitherto for decontaminating nuclear reactors and associated devices are described in detail as regards their mode of operation and special areas of application.
For the decontamination of water-cooled nuclear reactors built in the main from steel, the so-called "AP-Citrox" process has been the most successful of the known processes. In this process the contaminated metallic surfaces of the coolant circuits of nuclear reactors and the associated devices that have coolant flowing through them are first treated with an aqueous alkaline permanganate solution containing 3% potassium permanganate and 10% sodium hydroxide over several hours at a temperature in the region of 102.degree.-110.degree. C.; then, after rinsing with water an after-treatment process with a 10% aqueous solution of dibasic ammonium citrate is carried out, also over several hours and at a temperature of about 90.degree. C. By means of the treatment with the permanganate solution, the chromium(III) oxide, which is contained in the contamination layer in a much enriched amount compared with the chromium content of the steel beneath the layer and which in this form is only dissolvable with great difficulty, is transformed into chromium(VI) oxide which is well soluble in aqueous alkaline solutions according to the reaction equation EQU 2MnO.sub.4.sup.- +Cr.sub.2 O.sub.3 .fwdarw.2MnO.sub.2 +Cr.sub.2 O.sub.7.sup.--
and then the chromium(VI) oxide together with other components of the contamination layer that are soluble in alkaline solutions are dissolved by the aqueous alkaline permanganate solution from the contamination layer, in the course of which the previously relatively solid structure of the contamination layer changes into a looser porous condition. The after-treatment with the reducing ammonium citrate solution following the removal of the permanganate solution and rinsing with water dissolves the remaining contamination layer from the steel lying beneath it, and with it, radioactive materials still contained therein and dissolves them in the ammonium citrate solution.
However, it is a disadvantage of this known contamination process that the alkaline permanganate solution is a concentrated alkaline solution and thus brings with it the risk of corroding the cooling system of the reactor. This danger of corrosion is of considerable significance since leakages in the cooling system of nuclear reactors resulting from corrosion can have very grave consequences, which are not restricted only to outflow of coolant containing radioactive materials but also, as is is known, may lead to a fusion of the whole reactor and melt-through of the protective concrete sheath of the reactor plant, the so-called GAU. As is also generally known, to prevent a catastrophe associated with a GAU, extensive emergency cooling equipments are built into the more modern reactor plants, which take over the emergency cooling of the reactor in the event of a leak in the cooling system. Understandably, however, an attempt is made to eliminate, as far as possible, even the risk of a leak arising in the cooling system, and this again leads to the attempt to limit the danger of corrosion in the course of decontamination to a minimum. Such a demand, however, mulitates against the use of highly concentrated oxidising or reducing solutions for decontamination so that lately, particularly after the appearance of small leaks in the cooling systems of nuclear reactors that have been in opertion for a long time, matters have gone so far as to require users to content themselves with no decontamination at all, or with decontamination carried out with hot water only, which is wholly unsatisfactory, purely to avoid all risks.
It is highly disadvantgeous in such a wholly unsatisfactory decontamination process that in the course of the necessary inspection work in the cooling system of the reactor the personnel carrying out the work is exposed to a high radiation dose which causes damage to health and which in addition requires the provision of a plurality of work gangs and the replacement of the work gangs after only a very short working period in order to limit the effective exposure to radiation of the individual work gangs.
The corrosion risk involved in the use of highly concentrated alkaline or acidic solutions for decontamination is partly also caused by the fact that the duration of action of the highly concentrated solution on the contaminated metal surfaces within a reactor coolant circuit varies according to the position of such surfaces, since both the filling of the cooling system with the highly concentrated solution and the draining of the solution from the system require a certain time, respectively. The duration of action of the solution on parts at the deepest positions of the cooling circuit, with which during filling it first comes into contact and with which it breaks contact last, is thus greater than the duration of action on parts at the highest positions of the cooling circuit, with which it comes into contact last during filling and with which it breaks contact first during drainage. Since a certain minimum duration of action is required for decontamination and if one observes thus a minimum duration of action also for the highest-lying parts of the cooling system without wishing to exceed it so as to avoid unnecessary corrosion risks, then the most deeply lying parts of the cooling system are exposed to a minimum duration of action that is longer than the filling and drainage times. In other words, it then depends how large is the sum of filling and drainage times in relation to the minimum duration of action. If the minimum duration of action is relatively large and the sum of filling and drainage times is thus small relative to the minimum duration of action then the durations of action of the solution for the highest and deepest positions of the cooling system are approximately equal and about equal to the minimum duration of action. In other words, in this case the risk of corrosion is limited because the solution exerts its effect during the minimum duration of action principally on the contamination layer and changes to acting on the metal lying beneath the contamination layer only after the minimum duration of action has elapsed. If, on the other hand, the minimum duration of action is relatively small so that the sum of the filling and draining periods is of the same order of magnitude as the minimum duration of action, then the duration of action of the solution at the most deeply-lying locations of the cooling system is a multiple of the minimum duration of action. In this case, significant corrosion damage may arise at the more deeply-lying regions of the cooling system because the solution begins to act on the metal lying beneath the contamination layer even before the end of the minimum duration of action, and then for corroding the metal lying beneath the contamination layer there is available a time period of the same order of magnitude as, or greater than, the minimum duration of action. Since the minimum duration of action decreases with increasing concentration of the solution employed, there exists a real risk of corrosion for the more deeply-lying regions of the cooling system when using highly concentrated alkaline or acidic solutions for decontaminating because of the aforesaid filling and draining times (or the result is, when allowing the highly concentrated solution to act on the more deeply lying areas of the cooling system for the minimum duration of action only and thus avoids this risk of corrosion, that at the higher-lying areas of the cooling system there is only a highly inadequate decontamination or in certain circumstances even none at all). In contrast, when using low concentration or dilute solutions with a minimum duration of action considerably longer than the filling and drainage periods, the risk of corrosion is almost completely avoided by making the time from the beginning of the filling to the end of draining equal to the minimum duration of action without having to accept insufficient decontamination in the higher areas of the cooling system.
The use of highly concentrated alkaline or acidic solutions for decontamination has, in addition to the above-described disadvantage of significant corrosion risk (and the resulting danger of leakages in the cooling system and all the attendant consequences), the further disadvantage that after drainage from the cooling system, the processing of the solution necessary to remove radioactive substances dissolved therein from the contamination layer during the decontamination procedure is considerably more complicated and thus costly for concentrated than for dilute solutions.